The present invention relates generally to electrostatic latent image development, and, more particularly, concerns a multicolor image-on-image reproduction machine using a reverse charge printing (RCP) process.
Generally, processes for electrostatographic copying and printing are initiated by selectively charging and/or discharging a charge receptive image bearing member in accordance with an original input document or an imaging signal, generating an electrostatic latent image on the image bearing member. This latent image is subsequently developed into a visible image by a process in which charged developing material is deposited onto the surface of the latent image bearing member, wherein charged particles in the developing material adhere to image areas of the latent image. The developing material typically comprises carrier granules having toner particles adhering triboelectrically thereto, wherein the toner particles are electrostatically attracted from the carrier granules to the latent image areas to create a powder toner image on the image bearing member.
Alternatively, the developing material may comprise a liquid developing material comprising a carrier liquid having pigmented marking particles (or so-called toner solids) charge director materials dissolved therein, wherein the liquid developing material is applied to the latent image bearing image bearing member with the marking particles being attracted to the image areas of the latent image to form a developed liquid image. Regardless of the type of developing material employed, the toner or marking particles of the developing material are uniformly charged and are electrostatically attracted to the latent image to form a visible developed image corresponding to the latent image on the image bearing member.
The developed image is subsequently transferred, either directly or indirectly, from the image bearing member to a copy substrate, such as paper or the like, to produce a "hard copy" output document. In a final step, the image bearing member is cleaned to remove any charge and/or residual developing material therefrom in preparation for a subsequent image forming cycle.
The above-described electrostatographic printing process is well known and has been implemented in various forms in the marketplace to facilitate, for example, so-called light lens copying of an original document, as well as for printing of electronically generated or digitally stored images where the electrostatic latent image is formed via a modulated laser beam. Analogous processes also exist in other electrostatic printing applications such as, for example, ionographic printing and reproduction where charge is deposited in image-wise configuration on a dielectric charge retentive surface (see, for example, U.S. Pat. No. 4,267,556 and 4,885,220, among numerous other patents and publications), as well as other electrostatic printing systems wherein a charge carrying medium is adapted to carry an electrostatic latent image. It will be understood that the instant invention applies to all various types of electrostatic printing systems and is not intended to be limited by the manner in which the image is formed on the image bearing member or the nature of the latent image bearing member itself.
As described hereinabove, the typical electrostatographic printing process includes a development step whereby developing material is physically transported into contact with the image bearing member so as to selectively adhere to the latent image areas thereon in an image-wise configuration. Development of the latent image is usually accomplished by electrical attraction of toner or marking particles to the image areas of the latent image. The development process is most effectively accomplished when the particles carry electrical charges opposite in polarity to the latent image charges, with the amount of toner or marking particles attracted to the latent image being proportional to the electrical field associated with the image areas. Some electrostatic imaging systems operate in a manner wherein the latent image includes charged image areas for attracting developer material (so-called charged area development (CAD), or "write white" systems), while other printing processes operate in a manner such that discharged areas attract developing material (so-called discharged area development (DAD), or "write black" systems).
Image quality in electrostatographic printing applications may vary significantly due to numerous conditions affecting latent image formation as well as development, among various other factors. In particular, image development can be effected by charge levels, both in the latent image, as well as in the developing material. For example, when the charge on dry toner particles becomes significantly depleted, binding forces with the carrier also become depleted, causing an undesirable increase in image development, which, in turn, causes the development of the latent image to spread beyond the area defined thereby. Similarly, one problem affecting the control of image quality in ionographic devices involves a phenomenon known as "image blooming" resulting from the effect of previously deposited ions or charge on the path of subsequent ions directed to the charge retentive surface. This problem is particularly noticeable when printing characters and edges of solid areas, resulting in character defects, wherein blooming artifacts may include picture elements being displaced by 1-2 pixels in distance. Image blooming can also be caused by poor charge retention and/or charge migration in the electrostatic latent image on the latent image bearing member, a problem which is particularly prevalent in ionographic systems, wherein a focused beam ion source is utilized for image-wise charging of a dielectric latent image bearing member.
The present invention more particularly, concerns a multicolor image-on-image reproduction machine advantageously using a reverse charge printing (RCP) process, and the following disclosures may be relevant to some aspects of the present invention. U.S. Pat. No. 4,504,138 discloses a method of developing a latent electrostatic charge image formed on a photoconductor surface comprising the steps of applying a thin viscous layer of electrically charged toner particles to an applicator roller preferably by electrically assisted separation thereof from a liquid toner suspension, defining a restricted passage between the applicator roller and the photoconductor surface which approximates the thickness of the viscous layer, and transferring the toner particles from the applicator roller at the photoconductor surface due to the preferential adherence thereof to the photoconductor surface under the dominant influence of the electric field strength of the electrostatic latent image carried by the photoconductive surface, the quantity of toner particles transferred being proportional to the relative incremental field strength of the latent electrostatic image. An apparatus for carrying out the method of the invention is also disclosed, which includes an applicator roller mounted for rotation in a container for toner suspension, an electrode arranged adjacent the circumferential surface of the roller to define an electrodeposition chamber therebetween and electrical connections between the roller, the electrode and a voltage source to enable electrolytic separation of toner particles in the chamber, forming a thin highly viscous layer of concentrated toner particles on the roller.
U.S. Pat. No. 5,387,760 discloses a wet development apparatus for use in a recording machine to develop a toner image corresponding to an electrostatic latent image on an electrostatic latent image carrier. The apparatus includes a development roller disposed in contact with or near the electrostatic latent image carrier and an application head for applying a uniform layer of the wet developer to the roller.
U.S. Pat. No. 5,436,706 discloses an imaging apparatus including a first member having a first surface having formed thereon a latent electrostatic image, wherein the latent electrostatic image includes image regions at a first voltage and background regions at a second voltage. A second member charged to a third voltage intermediate the first and second voltages is also provided, having a second surface adapted for resilient engagement with the first surface. A third member is provided, adapted for resilient contact with the second surface in a transfer region. The imaging apparatus also includes an apparatus for supplying liquid toner to the transfer region thereby forming on the second surface a thin layer of liquid toner containing a relatively high concentration of charged toner particles, as well as an apparatus for developing the latent image by selective transferring portions of the layer of liquid toner from the second surface to the first surface.
U.S. Pat. No. 5,619,313 discloses a method and apparatus for simultaneously developing and transferring a liquid toner image. The method includes the steps of moving a photoreceptor including a charge bearing surface having a first electrical potential, applying a uniform layer of charge having a second electrical potential onto the charge bearing surface, and image-wise dissipating charge from selected portions on the charge bearing surface to form a latent image electrostatically, such that the charge-dissipated portions of the charge bearing surface have the first electrical potential of the charge bearing surface. The method also includes the steps of moving an member biased to a third electrical potential that lies between said first and said second potentials, into a nip forming relationship with the moving image bearing member to form a process nip. The method further includes the step of introducing charged liquid toner having a fourth electrical potential into the process nip, such that the liquid toner sandwiched within the nip simultaneously develops image portions of the latent image onto the member, and background portions of the latent image onto the charge bearing surface of the photoreceptor.